These New York City sisters are doing it for themselvesThough Christina Gandia Gambale and ­sister Jennifer Gandia officially took ownership of their parents’ 35-year-old jewelry store just a year ago, they’ve spent years putting their stamp on Manhattan’s Greenwich Jewelers. Located just blocks from the World Trade Center site, the store not only survived the 9/11 attacks, but also grew to become a destination boutique for New Yorkers and tourists alike. Greenwich is renowned for its selection: a well-curated mix of fashion (Alexis Bittar, ­Ben-Amun) and fine (Anne ­Sportun, Beverley K) pieces at a variety of price points. But thanks to the cyber-savvy siblings, it’s also known for extensive e-commerce offerings, ­Facebook flash sales, and numerous daily tweets.How have you two changed the family business since taking over your parents’ store? Christina: The most obvious d